Optical fibres are today used in numerous applications that span very diverse fields of optics. These fields include telecommunications, medicine, sensors, lasers, and many others. Photonic crystal fibres (PCFs) have recently emerged as an attractive class of fibres, where various properties may be tailored in new or improved manners compared to conventional (solid, non-micro-structured) optical fibers. PCFs are generally described by Bjarklev, Broeng, and Bjarklev in “Photonic crystal fibres”, Kluwer Academic Press, 2003.
An NA Converting Optical Waveguide Coupler
A common problem in fibre optics is to launch light into a fibre efficiently. Often the source of light and the fibre to couple into have different divergence angles (numerical aperture (NA)) and spot/core sizes. A specific problem is to launch light from a pump-diode-laser with a large spot size and relatively low numerical aperture into a double clad fibre laser with a small area and large numerical aperture.
The traditional method of solving this problem is to use bulk optics. This solution has a number of problems. One problem is related to difficulties in achieving coupling with low loss. Another problem is to achieve good coupling for a wide range of wavelengths. A third problem is mechanical stability. Fabrication of devices using bulk optics is also relatively complicated. Furthermore, reflection from the multiple glass surfaces may degrade performance of the system.
WO-2003/019257 deals with optical waveguides, for which improved coupling into cladding pumped optical fibres may be obtained through optimal designs of micro-structured outer cladding regions that provide high NA for mode(s) of an inner cladding region. This is achieved by the use of low index cladding features with a relatively narrow area between neighbouring low-index features constituting an air-clad surrounding the inner cladding.
An Optical Coupler Comprising a Bundle of Input Waveguides
About 10 years ago, a new family of optical fibres has appeared, called double cladding fibres. They consist of two waveguides imbedded into each other; an inner and an outer guiding region. Typically, the inner guiding region is a single mode core, whereas the outer region typically is a multi mode core, also called inner cladding.
Microstructured optical fibres, also known as Photonic Crystal Fibres (in the following called PCFs), holey fibres, hole-assisted fibres and by other terms, is a relatively new class of fibres where the guiding mechanism is provided by introducing air holes into the fibre. These holes typically run parallel with the fibre and extend all the way along the fibre length. The guiding principle can either be based on Total Internal Reflection (TIR) such as in traditional optical fibres, or the Photonic Bandgap (PBG) principle. For TIR-based fibres the waveguide (core) typically consists of solid glass having a larger refractive index than the effective refractive index of the surrounding cladding material, which includes a number of closely spaced holes.
In recent years, PCFs have been developed to also show double cladding features. Here, a ring of closely spaced air holes (air-clad) 13 will define the multi mode inner cladding (see FIG. 11). Fibres with air-clad and their fabrication are described in U.S. Pat. No. 5,907,652 and WO03019257 that are incorporated herein by reference. The Numerical Aperture (NA) is mainly given by the distance between these holes and can take values from below 0.2 all the way up to more than 0.8, although typical values lies around 0.6. The core at the centre is typically designed for single-mode operation although multi-mode is also used. In a PCF with a microstructured inner cladding 112, typically holes 111 are placed to lower the effective refractive index. The core 10 may be formed by leaving one or more holes near the centre (see FIG. 11). Alternatively, the core 220 can simply be defined by using a solid material 221 with a higher refractive index than the rest of the inner cladding. Again an air-clad is formed by a ring of holes 222 (see FIG. 12).
A typical use for double cladding fibres is to efficiently convert low quality, low brightness light from e.g. semiconductor lasers to high quality, high brightness light. Brightness is defined as optical power per solid angle per area. For multi mode fibres, conservation of brightness means that the NA multiplied with the waveguide diameter is a constant before and after the coupling/conversion.
The brightness conversion is done by doping the core with a rare earth dopant and pumping this with the multi mode light. The rare earth atoms will absorb the pump light and re-emit the energy at lower photon energies. Since the emission will happen through stimulated emission, this light will be guided in the mode core. Typically single mode operation is preferred, but multi-mode operation is also relevant.
This conversion method can be very efficient (up to around 80%) and the brightness can be improved by more than a factor of 100. Such light sources are often used as popular alternatives to high brightness solid state laser, since they are less bulky and far more efficient.
The limiting factor for the traditional fibres is the refractive index of the cladding material covering the inner cladding/pump guide. A low refractive index will result in a high NA of the pump guide. This, in turn, will allow either a smaller inner cladding diameter or the coupling of higher optical powers into the guide. Smaller inner cladding diameter means that the pump intensity is increased, which will allow higher efficiency and shorter laser cavities. The current state of the art for non-PCFs is to use a Fluorine-containing polymer cladding with a low refractive index. This will result in an NA of about 0.45. The problem with this is that such a polymer cladding often has problems tolerating the high optical powers, and will burn or degrade over time.
The PCFs, on the other hand, can achieve very high NAs and may be fabricated using only glass-based materials. This means that the inner cladding diameter can be reduced and that the thermal problems are alleviated. Also, there are further advantages, which will be outlined in the following.
When coupling light into a double-cladding PCF, there are a few considerations to make. To make full use of the high NA of the PCFs, one can use free space optics, such as lenses to couple the pump light into the inner cladding. An example can be seen in FIG. 13, where pump light from a single source, for example a fibre 30 delivering a pump light, is to be coupled into a single end of a PCF 31. The first (slow) lens 32 collimates the light 33 from the pump fibre, whereas the second (fast) lens 34 focuses the light into the inner cladding. This approach has the disadvantage that only one pump fibre can be used. Also, such a solution typically has only a coupling efficiency of 80-90%, has high reflections, is sensitive to mechanical drift and instability and sensitive to contamination. Finally, such solution makes packaging design for a commercial device complicated and expensive.
All-fibre pump multiplexers have been developed by several companies, such as ITF, OFS and Nufern. In such couplers, several pump fibres 40 are bundled together and heated to temperatures near melting and tapered 41 (see FIG. 14). Using a slow enough taper, the light from each pump fibre will merge and the down-taper of the diameter will slowly (adiabatically) increase the NA up to 0.45 or even higher.
The problem with these traditional fibre couplers is that the high NA (higher than 0.3) at the output presents a challenge which until now have not been solved. The object of the invention is thus to provide a fiber coupler for coupling two or more light sources which is improved with respect to the prior art fiber couplers, and in particular a fiber coupler which is improved with respect to low loss.
A Mode Field Converting Optical Fibre
Tapered optical fibres are used in a wide variety of optical applications including couplers and mode converters.
A tapered micro-structured fibre system is disclosed in US-2002/0114574. The micro-structured fibre has a core region, a cladding region and one or more axially oriented elements (e.g. capillary air holes) in the cladding region. In an embodiment, the axially oriented elements are partially or fully collapsed during heating and stretching, leaving a silica cladding in its place and thereby providing a mode expansion. This has the disadvantage of not providing a controllably confined mode field.
U.S. Pat. No. 6,778,562 deals with a coupler for a multimode pump comprising a photonic crystal fibre with a stretched portion and at least one multimode fibre coupled thereto. A disadvantage of this coupler is that the mode field diameter is smaller at the relatively smaller cross sectional end (the down-tapered end) of the tapered fibre than at the relatively larger cross-sectional end (the un-tapered end).
Double-clad fibres, e.g. optical fibres with an active core for amplifying an optical signal, an inner cladding for guiding multi-mode pump light and an outer cladding, are known. Such fibres receive a large interest due to their potential as high power amplifiers and lasers, see e.g. U.S. Pat. No. 5,907,652 or WO-03/019257. While these fibres have attractive properties, several practical difficulties exist. For example, many of the unique properties are related to fibres with large cores, so-called large-mode area fibres, including providing tapered fibre bundles with signal feed-through having large mode area at the reduced-diameter end. Simultaneously coupling of pump and signal light to such fibres is a problem. A common approach is to attach a tapered fibre bundle including a single mode fibre onto the double-clad fibre, such as disclosed in U.S. Pat. No. 5,864,644 or in U.S. Pat. No. 5,935,288. However, it is in practice difficult to accurately control the mode field diameter of the single mode core at the tapered end. In particular, it becomes increasingly difficult to control the mode field diameter, as it is increased compared to the un-tapered end, as the mode field diameter is highly sensitive with respect to the core size for these prior art optical fibres. As the MFD is expanded, it expands much faster than the simple scaling of the dimensions of the optical fibre. Further, a large variation in MFD is observed when the core size is reduced. In other words, great care is needed during tapering of the fibre to ensure correct dimensions. Therefore there is a need for improved tapered, optical fibres, specifically tapered fibres providing a smaller variation or more controllable expansion of MFD in the down-tapered end, thereby relaxing the tolerances of the dimensions of the resulting tapered fibre.